Molecular optical air data systems (MOADS)

ABSTRACT

A compact optical instrument called Molecular Optical Air Data System (MOADS) can directly measure wind speed and direction, density, and temperature of a body of air. From these measurements, a complete set of air data products can be determined. In contrast to pitot tubes, however, MOADS can operate at high angles of attack. In the proper configuration, MOADS can continue to measure air data products at angles of attack of 90 degrees. The MOADS instrument is a flush-mount design which lends itself to low observability since there are no aircraft protrusions to generate a radar cross section. MOADS is also airframe independent, and is much less costly to calibrate, recalibrate or service due to this lack of dependence. The system uses a Fabry-Pérot interferometer to detect the (incoherent) Doppler shift from laser light backscattered by air molecules and aerosols (Rayleigh and Mie scattering). The laser used to provide the signal utilizes short wavelengths operating in the ultraviolet at 266 nm, which is invisible to the human eye and rapidly absorbed by the atmosphere. Although the system will take advantage of aerosols when they are available, a significant advantage of MOADS over similar air data system technologies is the ability to make measurements in clear air (air molecules only), without the presence of aerosols, which also provides the ability to measure density and temperature.

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional PatentApplication Serial No. 60/360,818, filed Mar. 1, 2002, the entirecontent of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to air data systems and, inparticular, to an optical instrument that can directly measure windspeed and direction, density, and temperature to derive a complete setof air data products.

BACKGROUND OF THE INVENTION

[0003] “Air data products,” as they are called, are determined in anaircraft using an in-flight air data system. An air data systemincorporates instrumentation to collect air data products, and suppliesthis data directly to an aircraft's flight computer for flight controlpurposes. Common air data products include, but are not limited to, Machnumber, true airspeed, calibrated airspeed, vertical speed, staticdensity, static air temperature, sideslip, angle of attack, pressurealtitude, and dynamic pressure.

[0004] Perhaps the oldest type of such instrumentation is the Pitotstatic tube. The Pitot tube (named after Henri Pitot in 1732) measures afluid velocity by converting the kinetic energy of the flow intopotential energy. The conversion takes place at the stagnation point,located at the Pitot tube entrance. A pressure higher than thefree-stream (i.e. dynamic) pressure results from the kinematic topotential conversion. This “static” pressure is measured by comparing itto the flow's dynamic pressure with a differential manometer.

[0005] Pitot static tubes have proven quite effective over the years;however, there are a number of characteristics that make themundesirable in some situations. For example, at high angles of attackthe air data measurements provided by pitot static systems aresignificantly degraded. Pitot tubes also contribute significantly to anaircraft's radar cross section, since they protrude from the aircraftbody. The installation and calibration of pitot static tubes must betailored to each airframe, and airframe modifications may requirerecalibration of the air data system.

[0006] Optical air data system technologies are alternatives to thetraditional pitot static system. In general, an optical air data systemutilizes LIDAR (Light Detection and Ranging) to remotely analyze theatmosphere. LIDAR uses an active sensor that includes a laser lightsource, a detection system and an analysis routine to process the signalreturn.

[0007] There are two types of optical air data systems: coherent anddirect detection (incoherent). In a coherent LIDAR, the laser light isemitted into the atmosphere, where it scatters off of the aerosols inthe air, and can be analyzed to solely determine the air velocity. Forthese purposes, an aerosol is defined as any type of particle that issuspended in the air.

[0008] In a direct detection LIDAR, the laser energy scatters off ofboth aerosols in the air, as well as the air molecules themselves, andcan be analyzed to determine the air velocity, density, and temperature.A schematic of a Doppler LIDAR system configuration is presented inFIG. 1. This figure illustrates how transmitted laser light, which hasbeen scattered by the atmosphere, is gathered by a telescope andprocessed by the detection system.

[0009] A coherent LIDAR system utilizes long wavelength laser energy andrelies upon Mie scattering, which is the scattering of light off of theaerosols suspended in the air. More particularly, Mie scattering refersto the scattering of light off of particles greater than {fraction(1/10)}^(th) the wavelength of light. However, since coherent detectionLIDAR measures the properties of aerosols, it can only measure the windvelocity.

[0010] A direct detection LIDAR system utilizes short wavelength laserenergy in order to scatter the light off of both the molecules of air(Rayleigh scattering) as well as the aerosols in the air (Miescattering). Rayleigh scattering refers to the scattering of light offof molecules of air, and particles up to {fraction (1/10)}^(th) thewavelength of the light. Since direct detection LIDAR measures theproperties of molecules, it can measure the air velocity, as well as theair density and temperature.

[0011] However, since coherent LIDAR approaches rely solely on Miescattering, they cannot make measurements in clean air where there areno aerosols present. In addition, coherent approaches utilize longwavelength light, which is not absorbed by the atmosphere, presentingadditional issues with long-range detection, and increased eye safetyhazards.

SUMMARY OF THE INVENTION

[0012] This invention, termed the Molecular Optical Air Data System(MOADS), is a compact, direct detection LIDAR optical instrument thatcan directly measure wind speed and direction, density, and temperatureof a body of air. From these measurements, a comprehensive set of airdata products can be determined.

[0013] In some respects, MOADS is a replacement for pitot static tubes.In contrast to pitot tubes, however, MOADS can operate at high angles ofattack. In the proper configuration, MOADS can continue to measure airdata products at angles of attack of 90 degrees. The MOADS instrument isa flush-mount design which lends itself to low observability since thereare no aircraft protrusions to generate a radar cross section. MOADS isalso airframe independent, and is much less costly to calibrate,recalibrate or service due to this lack of dependence.

[0014] In terms of apparatus, MOADS uses a Fabry-Pérot interferometer todetect the (incoherent) Doppler shift from laser light backscattered byair molecules and aerosols (Rayleigh and Mie scattering). The laser usedto provide the signal utilizes short wavelengths operating in theultraviolet at 266 nm, which is invisible to the human eye and rapidlyabsorbed by the atmosphere.

[0015] Although the system will take advantage of aerosols when they areavailable, a significant advantage of MOADS over similar air data systemtechnologies is the ability to make measurements in clear air (airmolecules only), without the presence of aerosols.

[0016] The advantages of the MOADS instrument include the following:

[0017] Low observability

[0018] Operates at high angles of attack (in the proper configuration,MOADS can continue to measure air data products at angles of attack of90 degrees)

[0019] Operates in clear air (aerosols are not required)

[0020] Airframe independent

[0021] Less costly to calibrate or recalibrate

[0022] Accurate for highly maneuverable aircraft as well as hoverableaircraft

[0023] Reduced Life Cycle Cost

[0024] Possible application to detection of wind shear, wake vortex,clear air turbulence, and engine unstart conditions

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a schematic illustrating a Doppler LIDAR concept;

[0026]FIG. 2 is a diagram of an etalon optical system according to theinvention;

[0027]FIG. 3A shows normal Etalon fringes;

[0028]FIG. 3B depicts an etalon illuminated with four fiber inputchannels;

[0029]FIG. 4 shows how the four channels of fringes are collapsed tofour lines in the shape of a cross pattern on an opto-electric detector;

[0030]FIG. 5 is a MOADS opto-mechanical system diagram;

[0031]FIG. 6 is a fringe analysis diagram;

[0032]FIG. 7 shows MOADS measured air data products; and

[0033]FIG. 8 shows MOADS derived air data products.

DETAILED DESCRIPTION OF THE DRAWINGS

[0034] The MOADS (Molecular Optical Air Data System) direct detectionLIDAR system described herein is unique in that it can operate in clearair, using only molecular backscatter (i.e., no aerosols present). TheMOADS system takes advantage of aerosols when present, but does not relyupon the presence of aerosols. The signal generated by MOADS is used todirectly measure velocity, true airspeed, vertical speed, angle ofattack, angle of sideslip, static density and static temperature. Fromthese data products the following can be directly calculated: calibratedairspeed, Mach number, static pressure, total pressure, dynamicpressure, pressure altitude, air density ratio, total temperature, angleof attack, pressure differential, and angle of sideslip pressuredifferential.

[0035] In the preferred embodiment, a beam of ultraviolet laser light ata wavelength of 266 nm is emitted in three directions from asurface-mounted aperture in the aircraft, and to detect the return fromscattering of this beam by atmospheric molecules and aerosols. Theinitial wavelength of 266 nm was selected for its stealth, eye safetyand molecular scattering properties. There is very little naturalbackground light due to absorption of most natural 266 nm light by ozoneand molecular oxygen. However, if longer wavelength light were to beused, a simple gating system would eliminate most of the stray light.Ultraviolet (UV) light at 266 nm is readily absorbed by glass andplastic, such as aircraft wind screens.

[0036] Although the invention is described herein with respect to anairframe-mounted unit, it must be appreciated that the invention canmeasure air data products on a variety of platforms, not limited to anaircraft proper. Other possibilities include, but are not limited to,smart-guided weapons, stationary weather stations (most notably on icymountain tops), and possibly wind-propelled boats. In addition, althoughthe current prototype uses ultraviolet (UV) laser light, the system canoperate on large range of wavelengths spanning from the visible down tothe ultraviolet. The UV light provides additional stealthcharacteristics for the system since the light is quickly absorbed bythe atmosphere, and not easily detected from long-range distances.However, MOADS can also operate in other wavelength regions, such aslonger UV wavelengths or even visible wavelengths. The laser source canbe either pulsed or continuous wave (CW).

[0037] Nor is the invention limited in terms of the number of channels,nor the geometry of the channels in relation to each other. Although thesystem described herein is configured with three channels, spaced 120degrees apart from each other, other angles may be used to calculate awind vector. In addition, although three channels are necessary tocalculate a wind vector, the system may have extra redundant channels,or dual channels to measure wind in a particular plane, or singlechannels to measure the wind along a specific light of sight.

[0038] An aspect that sets MOADS apart from other direct detectionoptical air data systems is the efficient use of the etalon. In thepreferred embodiment, a single Fabry-Pérot etalon is used for fourchannels of light: three lines of sight, plus a reference channel. FIG.2 is a diagram of the etalon optical system used by the invention. Afiber optic illuminates a portion of the etalon, producing the image ofa partial ring pattern shown in FIG. 3A. Note that, for the purposes ofclarity, FIG. 2 traces only the path associated with one of the fibers;in the actual system implementation, four fiber inputs are used. FIG. 3Bdepicts an etalon illuminated with four fiber input channels.

[0039] The reference channel is injected directly from the laser source.All three lines of sight are compared to the reference channel, toproduce an instrument that is self-calibrating by design. If laserwavelength drift is not accounted for in the data, then errors willarise when making a measurement of the Doppler shift (wavelength shift)of the signal return. In the MOADS instrument, however, the laserwavelength drift is automatically calibrated out of the data since eachmeasurement is compared to the reference light source.

[0040] A quad circle-to-line interferometer optic (CLIO) is preferablyused to collapse the four channels of circular fringes down to fourlinear patterns, forming the shape of a cross. Although use of the CLIOis not strictly necessary to the invention, its use does provideadditional signal throughput. The resultant cross pattern is much easierto detect using standard linear-based detectors, such as linear arraysor CCDs. The CLIO, described in U.S. Pat. No. 4,983,033, the entirecontent of which is incorporated herein by reference, employs aninternally conical reflector for converting the circular fringeinformation into linear information whereby the reflected light can bereceived by conventional linear array detectors, such as charge coupleddevice which are used in spectroscopic analysis.

[0041] The azimuthal angle of the detected circular fringe pattern isreduced with the use of a tele-kaleidoscope having a predeterminedarrangement of mirrors. Electromagnetic energy which is issued from theinterferometer is propagated substantially along the conical axis of thecone of which the reflector forms a segment, and is reflected andfocused substantially onto a line in the vicinity of the conical axis,where the linear detector is situated. In a preferred embodiment, theapex of the cone is situated where the conical axis intersects the focalplane of the circular fringe pattern. FIG. 4 shows how the four channelsof fringes are collapsed to four lines in the shape of a cross patternon an opto-electric detector.

[0042] Important hardware aspects of the MOADS system are shown in FIG.5. The optical head is responsible for directing the outgoing laserbeam, as well as collecting the backscattered signal utilizing threeseparate telescopes. The optical head can be custom configured, butshown here at the center of the optical head, the laser is divided intothree beams with a beam splitter optic, and then directed into threelines of sight, each spaced 120 degrees from each other and 30 degreesoff of the center axis. The signal is then collected by an array ofthree telescopes built into the head.

[0043] It should be noted that scattering is preferably only detected inthe regions where the field of view of the detecting telescope and thelaser beam overlap. This creates a near-field region where there is noreturn, which is a desirable feature since the airstream near theaircraft is turbulent. The far-field measurement is not as contaminatedby the aircraft's wake. The signal from the detecting lens system iscollected by a fiber optic that transfers the returned photons to theinterferometer/detection system.

[0044] The system uses the geometry of the laser beam and the telescopeobservation system to define the range from the sensor, rather thanemploying timing as is done with most LIDAR systems. This is analogousto the situation encountered by passive sensing space flightinstruments, where the return signal is integrated along the line ofsight. Such an approach simplifies the system, but range gating couldeasily be introduced if desired.

[0045] The signal conditioner employs eight bandpass filter mirrorscentered around 266 nm which filter out the background light. The filterbox performance exhibits high out-of-band rejection, as well as lowin-band attenuation. The signal delivered to the etalon consists ofvirtually pure 266 nm wavelength light.

[0046] The interferometer is made-up of a Fabry-Pérot etalon, as well assome steering optics to focus the image onto the detector. The signalreceived in any one of the three interaction regions is processedthrough the Fabry-Pérot interferometer that acts as a comparator todetermine the wind speed, temperature, and air density. The techniquesfor determining these quantities were fully developed for the satelliteinstruments that have been flown since the early 1980s. Basic systemcalibration is maintained by performing simultaneous observations of theemitted laser beam with every observation of the three interactionregions.

[0047] The circular fringes generated by the Fabry-Pérot etalon aretransformed into a linear cross-pattern and then imaged onto acharge-coupled device (CCD) detector. The CLIO device referenced abovegreatly improves the efficiency of the signal detection process. The CCDcamera is low-light sensitive, and provides low noise image readout.

[0048] MOADS Data Analysis

[0049] Wind velocity, density, and temperature are directly calculatedfrom the Fabry-Pérot etalon fringe data. All other air data products arederived from these three basic measurements, and the knowledge of theinstruments geometry. FIG. 6 is a fringe analysis diagram.

[0050] Velocity Measurement

[0051] The relative wind velocity is determined by the differencebetween the centroids of the atmospheric fringes versus the laserreference fringes. The fringe position is based directly on wavelength.Therefore the difference in wavelength between the signal and referencefringes is a direct measure of the molecular/aerosol Doppler shift.

[0052] Density Measurement

[0053] The air density is determined by the integral of the molecularcomponent of the atmospheric fringes. Air density is related to themolecular density, not aerosol density, therefore the Rayleighbackscatter must be separated from the Mie backscatter. The denser theair is, the more molecules are present to scatter light back to thedetector. The density measurement is a measurement of the total numberof photons received.

[0054] Temperature Measurement

[0055] The absolute temperature is determined by the width of themolecular component of the atmospheric fringes. The air temperature isrelated to the vibrational mode of the molecular content. As themolecules exhibit a faster vibrational state, the backscatter bandwidthis widened, producing wider fringes. The absolute temperature isdirectly related to this signal bandwidth.

[0056] Calculation of the Air Data Products

[0057]FIGS. 7 and 8 illustrate the transformation of instrumentline-of-sight data to measured and derived air data products.

We claim:
 1. An optical air data system, comprising: a laser outputtingone or more beams of light; an optical head for directing the beams oflight into the atmosphere, and to collect the light backscattered bymolecules or aerosols present in the atmosphere; an interferometeroperative to generate an atmospheric fringe pattern associated with thelight backscattered along multiple channels; a sensor to detect theincoherent Doppler shift in the fringe patterns and output correspondingelectrical signals; and a data processor to determine a set of air dataproducts based upon the electrical signals.
 2. The optical air datasystem of claim 1, wherein the interferometer is based upon aFabry-Pérot etalon.
 3. The optical air data system of claim 1, whereinthe air products include wind speed, temperature, and air density. 4.The optical air data system of claim 1, further including acircle-to-line interferometer optic to transform the fringe patternsfrom circular pattern to a linear cross-pattern.
 5. The optical air datasystem of claim 1, further including a reference beam from the laserinto the interferometer to calibrate the optical channels.
 6. Theoptical air data system of claim 1, further including telescopesassociated with the collection of the backscattered light with respectto each optical channel.
 7. The optical air data system of claim 1,wherein the backscattered light is only detected in the regions ofoverlap with the outgoing beams.
 8. The optical air data system of claim1, wherein the laser beams are in the ultraviolet region of thespectrum.
 9. The optical air data system of claim 1, wherein the laserbeams are in the visible region of the spectrum.
 10. The optical airdata system of claim 1, wherein the interferometer is operative todetect Rayleigh and Mie backscattering.
 11. The optical air data systemof claim 1, further including a reference fringe pattern deriveddirectly from the laser; and wherein the data processor is operative todetermine relative wind velocity by comparing the atmospheric fringepattern to the reference fringe pattern.
 12. The optical air data systemof claim 1, wherein the data processor is operative to determine airdensity in accordance with the integral of the molecular component ofthe atmospheric fringes.
 13. The optical air data system of claim 1,wherein the data processor is operative to determine absolutetemperature in accordance with the width of the molecular component ofthe atmospheric fringes.